1. Field of the Invention
The present invention relates to a combined cycle power plant in which a gas turbine plant and a steam turbine plant are combined, and to a cooling steam supply method for a gas turbine in operation thereof.
2. Description of the Prior Art
A combined cycle power plant is a power system in which a gas turbine plant and a steam turbine plant are combined, wherein a gas turbine takes charge of a high temperature section of thermal energy and a steam turbine takes charge of a low temperature section thereof. Therefore, the thermal energy is recovered for an effective use thereof, and this power system is now being paid a high amount of attention.
In the combined cycle power plant, research and development are being carried out aiming at a higher temperature gas turbine as one means for enhancing the efficiency.
On the other hand, in order to attain such a higher temperature, improvement of the cooling system which takes account of heat resisting ability of turbine structural elements must be pursued. As a result of various tries and errors, a steam cooled system which uses steam as the cooling medium, instead of a prior art use of compressed air, is in progress now.
One example thereof is the Japanese laid-open patent application No. Hei 05(1993)-163960, wherein a cooling steam is obtained from intermediate pressure steam of a waste heat recovery boiler. However, a sufficient volume of steam is not obtainable which results in difficulty in achieving a stable and secure cooling.
Thus, as a further progress thereafter, in order to achieve a sufficient volume and a stable cooling, a cooling system using exhaust steam from a high pressure steam turbine as the cooling steam is now being developed.
In the prior art steam cooled system as mentioned above, progress has been made from the system using intermediate pressure steam as the cooling medium to that using high pressure exhaust steam, and thus the practicality has been enhanced further. However, the high pressure exhaust steam is of a high temperature at the same time, hence a gas turbine high temperature portion (which is a portion to be cooled) must be made of a selected material which can resist such a high temperature.
A high temperature resistible material, being required to be of properties necessary for that material, becomes very expensive. Furthermore, in case of a turbine disc among others for example, there is a large difficulty in obtaining an appropriate material therefor within limited conditions of price and the like, which leads to a serious problem in the design and manufacture of the plant.